nixos/test-driver: remove irrevelant comment

BaseStartCommand is no longer thing.

nixos/lib/test-driver/src/test_driver/machine/__init__.py

@@ -194,8 +194,7 @@ class StartCommand:
 
 
 class NixStartScript(StartCommand):
-    """A start script from nixos/modules/virtualiation/qemu-vm.nix
-    that also satisfies the requirement of the BaseStartCommand.
+    """A start script from nixos/modules/virtualiation/qemu-vm.nix.
     These Nix commands have the particular characteristic that the
     machine name can be extracted out of them via a regex match.
     (Admittedly a _very_ implicit contract, evtl. TODO fix)